Magnetic resonance tomography scanners are imaging devices that, in order to depict an examination object, align nuclear spins of the examination object with a strong outer magnetic field and by way of a magnetic alternating field execute the same for precession about this alignment. The precession or return of the spins from this excited state into a state with less energy in turn generates, as a response, a magnetic alternating field that is received via antennas.
With the aid of magnetic gradient fields, a spatial encoding is imprinted on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated and a three-dimensional imaging depiction of the examination object is provided. The depiction generated indicates a spatial density distribution of the spins.
Diffusion of substances in the examination object, (e.g., water or hydrogen-containing substances), is described by a diffusion tensor, which may be depicted as a symmetric diffusion matrix with 3 ×3 components, 6 of which are independent. To determine the tensor, in the prior art, first an image of the density distribution of the substance to be examined, (e.g., hydrogen), is generated. This is the case, for example, with conventional magnetic resonance tomography. For determination of the individual components of the diffusion tensor, it is then necessary, to acquire the change in proton density with time quantitatively due to drift in different directions. To this end, after excitation by a gradient field, the protons are subject to location- and direction-dependent phase modulation and the phase modulation is reversed after a measuring interval. In the case of protons, which have then left the original location in the direction of the encoding, for example due to diffusion, the phase modulation reversal is incomplete and the protons then contribute with reduced intensity to the subsequently determined density distribution. This reduction and, from this, a component of the tensor may be determined by quotient formation.
Overall, in addition to unencoded density measurement, 6 further measurements with different encoding directions are necessary for the determination of all components of the diffusion tensor. Due to the small change in density due to diffusion, the signal-to-noise-ratio, SNR) in the measurements is much worse than with simple density measurements. Therefore, to produce useful measurements, it is necessary to take a plurality of measurements and calculate the average thereof thus significantly extending the measuring time.